1. Field of the Invention
The present invention relates to a writing method for a magnetic memory cell and a magnetic memory array structure.
2. Description of Related Art
Magnetic memories, such as magnetic random access memories (MRAMs), are non-volatile memories combining attributes of non-volatility, high density, high read and write speed, radiation resistance, and so on. Through magnetization directions of magnetic materials in neighboring tunnel barrier layers, the magnitude of magneto-resistance arisen from parallel or anti-parallel orientation is determined, and data 0 or 1 can be stored in the MRAM cells. Writing data into a selected magnetic memory cell is generally accomplished by passing currents through two current lines e.g. a bit line (BL) and a write word line (WWL) which intersect at the selected magnetic memory cell and induce a magnetic field. Simultaneously, through adjusting the magnetization direction of a free layer, the magneto-resistance is changed. Besides, when recalling the stored data, the current flows into the selected magnetic memory cell unit, and the resistance of the cell is detected to determine the digital value of the stored data.
FIG. 1 depicts a basic structure of a magnetic memory cell. Please refer to FIG. 1. Transverse current lines 100 and 102 with an adequate current flowing there through are required for accessing the magnetic memory cell. The current lines 100 and 102 may be described as the WWL and the BL based on the operating manner, such that the memory cells in a two-dimensional array can be controlled, respectively.
As the currents flow into the two conductive lines, the magnetic field with two directions is generated. Thereby, the magnetic field having the desired magnitude and direction can be applied to a magnetic memory cell 104. The magnetic memory cell 104 has a stack-layered structure including a magnetic pinned layer having a fixed magnetization or a fixed total magnetic moment in a predetermined direction. The data can be recalled based on the magnitude of the magneto-resistance. In addition, the data stored in the memory cell can be read out through output electrodes 106 and 108. Other details of the magnetic memory will not be provided hereinafter as they may be deduced by those of ordinary skill in the art.
FIG. 2 depicts a memory mechanism of a magnetic memory. As shown in FIG. 2, a magnetic pinned layer 104a has a fixed magnetic moment direction 107. A magnetic free layer 104c is disposed over the magnetic pinned layer 104a and is isolated therefrom by a barrier layer 104b sandwiched in between. The magnetic free layer 104c has a magnetic moment direction 108a or 108b. Since the magnetic moment direction 107 is parallel to the magnetic moment direction 108a, the magneto-resistance generated thereby represents the data “0”, for example. On the contrary, the magnetic moment direction 107 and the magnetic moment direction 108b are anti-parallel, and thus the magneto-resistance generated thereby refers to the data “1”, for example.
In general, the single-layered magnetic free layer 104c as shown in FIG. 2 may give rise to access errors. In order to avert disturbance caused by the adjacent memory cells when the data are written, a triple-layered free layer with a ferromagnetic (FM) layer/non-magnetic (M) metal layer/ferromagnetic (FM) layer structure is conventionally adopted as a solution to replace the single-layered FM material and to form a magnetic free stack layer 166 whose structure is indicated in FIG. 3. Anti-parallel FM metal layers 150 and 154 are respectively disposed on and under an M metal layer 152, and thereby a confined magnetic line is constituted. An underlying magnetic pinned stack layer 168 is isolated from the magnetic free stack layer 166 by a tunnel barrier (T) layer 156. The magnetic pinned stack layer 168 includes a top pinned (TP) layer 158, an M metal layer 160, and a bottom pinned (BP) layer 162. The TP layer and the BP layer both have a fixed magnetization. In addition, a base layer 164 e.g. an antiferromagnetic layer is disposed at the bottom.
In the triple-layered magnetic free stack layer 166, a magnetic anisotropic axis i.e. an easy axis is inclined at an angle of 45 degree with respect to both the BL and the WWL. Thereby, the BL and the WWL are capable of sequentially applying a magnetic field to the free stack layer 166 at 45 degrees from the easy axis, so as to rotate the magnetization of the free stack layer 166. The data stored in the memory cell are determined by the two magnetization directions of the FM metal layer 154 and the BP layer 158.
Further, it has been proposed in the related art to rotate the magnetization of the free layer in a toggle mode operation other than replacing the free layer having the single-layered free layer with the free layer having the triple-layered structure. FIG. 4 depicts an effect induced by an applied magnetic field on a triple-layered structure. Referring to FIG. 4, a thick arrow represents the applied magnetic field, and the length of the thick arrow denotes the magnitude of said applied magnetic field. Two thin arrows refer to two magnetization directions of the top FM layer and the bottom FM layer in the free stack layer. As the applied magnetic field is subtle, the two magnetization directions remain unchanged. When the applied magnetic field is increased to a certain magnitude, the two magnetization directions form an opening angle. And, when the applied magnetic field is excessively large, the two magnetization directions are drawn to the same direction as that of the applied magnetic field. The toggle mode operation complies with the second condition described above.
FIG. 5 depicts a time sequence of an applied magnetic field in a toggle mode. Referring to FIG. 5, H1 and H2 represent two directions of the applied magnetic fields at a 45 degree angle with respect to a direction of the easy axis, while two arrows in an ellipse indicate the two magnetization directions. At a time t0, the two magnetization directions are in the direction as the direction of the easy axis in the absence of the applied magnetic fields. Thereafter, H1 and H2 begin to rotate according to the time sequence illustrated in FIG. 5, and the total magnetic fields at different times (t1˜t3) are obtained. The two magnetization directions are then rotated. At a time t4, the application of the magnetic field is terminated. Here, the two magnetization directions are rotated once. That is to say, the data stored in the memory cell are modified through a write-in operation.
Moreover, since the write-in current is relatively high in the toggle mode operation, the related art has also brought up a bias field design. FIG. 6 is a schematic view depicting a reduction in an operating current according to the related art. Referring to the left graph in FIG. 6, the basic architecture of the memory cell is similar to that depicted in FIG. 3. The main difference lies in that a total magnetic moment 180 of the BP layer 162 is increased with respect to a total magnetic moment 182 of the TP layer 158. The total magnetic moment 180 is increased in thickness, for example. Since the total magnetic moments 180 and 182 are not balanced, a fringe magnetic field is generated, and a bias field 184 is applied to the free stack layer 166. Thereby, a toggle operating region in a first quadrant moves to a zero field, resulting in a small writing field 186. And because the required write-in magnetic field is small, the write-in current for generating the magnetic field can then be reduced.
The conventional operating modes have improved the write-in mechanism by which the data are written into the corresponding magnetic memory cell. However, the write-in method proposed by the related art can be merely performed on one magnetic memory cell, and the easy axis of the magnetic memory cell is pre-tilted in a 45 degree direction. Therefore, as the currents flow into the two write-in current lines, the direction of the magnetic field generated thereby is oriented at a 45 degree angle to the direction of the easy axis. If the direction of the easy axis is deflected during the fabrication of the magnetic memory cell, and if an excessive bias field is applied, some of the magnetic memory cells, in which the data stored not to be changed may be switched and may result in change. In view of the foregoing, optimizing the write-in efficiency and preventing an abnormal write-in operation are vital in the MRAM industry.